1. Field of the Invention
The present invention relates to a method and an apparatus for detecting a failure in a solar cell module in a photovoltaic power generation system, and to a solar cell module.
2. Related Background Art
Because of the recently increased concern for the environmental and energy issues developing in the global scale, the photovoltaic power generation system is attracting attention as a clean energy source. For installation, the solar cell is constructed as a ground installation type for installation on the field or a roof installation type for installation on the house roof, but the latter capable of effectively utilizing the roofs of the houses is mainly used because the installation of solar cells requires a large area. The roof installation type can be principally divided into a type for installation on a support and a type integrated with the roof material, but recently there is increasing the roof material-integrated solar cell module in which solar cells are stacked on a roofing steel plate and which has the function of the roof material.
In constructing a photovoltaic power generation system, a solar cell array is constructed by understanding the characteristics of the solar cell module and determining the number of series and parallel connections of the solar cells in order to obtain a desired output thereof. FIG. 2 is a block diagram of a general photovoltaic power generation system.
A solar cell array 20 is produced by connecting a plurality of solar cell modules in series to form a string and then by connecting a plurality of strings in parallel. In the photovoltaic power generation system, a direct current from the solar cell array 20 is collected at a junction box 50, then supplied to an inverter 60, and further supplied to a load 40 through a power distribution board 70. In the photovoltaic power generation system for ordinary house, the junction box 50 and the inverter 60 are often installed indoors for facilitating maintenance and inspection. Each solar cell module is provided with a bypass diode, and, in the case of an unbalance in the currents of the solar cell modules, the current bypasses the solar cell module to flow in the diode. In order to achieve the safe and efficient operation of the photovoltaic power generation system, it is essential to exactly detect the failure in the solar cell module.
For detecting a failure in a solar cell module, there are conventionally known, for example, a method of measuring the terminal voltage of solar cell modules with a tester and finding out the solar cell module of a lowered terminal voltage, a method of employing a light emitting diode as a bypass diode provided in a solar cell module and inspecting the light emitting state of the diode as disclosed in Japanese Patent Laid-Open No. 8-97456, and a method of providing means for changing color by a current flowing in a bypass diode of the solar cell module, in a solar cell module, as disclosed in Japanese Patent Laid-Open No. 9-148613.
However, in the above-mentioned method of measuring the terminal voltage of the solar cell, it is necessary to measure the terminal voltage of each solar cell module with a voltage detecting device such as a tester, and such measurement is very difficult particularly in the case the solar cell module is installed on the roof, because the wirings of the solar cell module are usually formed on the back face thereof.
Also in the method of detecting a failed solar cell module by employing a light emitting diode as the bypass diode provided in the solar cell module as disclosed in Japanese Patent Laid-Open No. 8-97456, a current flows in the light emitting diode even in a partial shade state in which the solar cell module is partially shaded, whereby the light emitting diode is turned on. Also the forward voltage drop of the light emitting diode is much larger than that in the ordinary diode. Therefore, the light emitting diode generates a large loss and wastes a precious power generated in the solar cell.
Furthermore, in Japanese Patent Laid-Open No. 9-148613, a current flowing in the bypass diode of the solar cell module is detected by a color changing member incorporated in the solar cell module and capable of changing color by a temperature change, and a failure in the solar cell module is found out by inspecting a color change in a single color changing member. However, the temperature of the solar cell module may rise to about 80xc2x0 C. under the strong solar radiation energy in the mid summer time, and it is impossible to find out a failure in the solar cell module by merely observing a color change in a single color changing member, in consideration of a case where a temperature rises to 80xc2x0 C. in the bypass diode connected to a normal solar cell module in summer and a case where a temperature rises to 80xc2x0 C. in the bypass diode by a failure in the solar cell module.
Also among the failures of the solar cell, there is known short circuit failure caused by the short circuit of the solar cell. In the solar cell with the short circuit failure, no current flows in the bypass diode, so that the short circuit failure cannot be detected solely by the conventional method of detecting a current flowing in the bypass diode.
An object of the present invention is to provide a method and an apparatus for detecting a failure in a solar cell module, and a solar cell module which allow to easily detect the failure in the solar cell module even when it is installed on a roof. Another object of the present invention is to enable exact detection of the failure in the solar cell module even in the case of employing the color changing member incorporated in the solar cell module and capable of changing color by a temperature change. Still another object of the present invention is to enable easy detection of the short circuit failure of the solar cell module.
In order to attain the above-mentioned objects, the method of the present invention of detecting a failure in a solar cell module comprises detecting the temperature of a bypass diode of the solar cell module from the exterior of the solar cell module and detecting the presence or absence of the failure therein based on the result of the temperature detection. Also the apparatus of the present invention for detecting a failure in a solar cell module comprises a temperature detecting means for detecting the temperature of a bypass diode of the solar cell module from the exterior of the solar cell module.
Thus, when the solar cell module is installed, for example, on the roof, the failure in the solar cell module can be easily detected from the top side thereof even if the solar cell module does not have the temperature detecting means therein.
Also, another method of the present invention of detecting a failure in the solar cell module comprises covering at least one solar cell in the solar cell module with a light shielding plate, detecting a current in a bypass diode bypassing the covered solar cell, and detecting the failed solar cell module based on the result of the current detection. Further, another apparatus of the present invention for detecting a failure in a solar cell module comprises a light shielding plate for covering the solar cell module or a solar cell, a current detection means for detecting a current in a bypass diode bypassing the solar cell module or the solar cell, and means for detecting a failed solar cell module based on the output from the current detection means.
It is thus possible to easily and exactly detect the solar cell module with a short circuit failure by utilizing a current flowed in the bypass diode when a normal solar cell module or a normal solar cell is shielded from light.
Further, the solar cell module of the present invention comprises a temperature detection means provided in the exterior of the solar cell module and capable of detecting the temperature of a bypass diode therein and the temperature of an interior of the solar cell module other than the diode.
When there is employed a color charging member incorporated in the solar cell module and capable of changing color by the temperature change, the temperature of a bypass diode can be evaluated in relative comparison with the temperature in the surrounding solar cell module under a similar environment, whereby the failure in the solar cell module can be accurately detected.
Furthermore, the solar cell module of the present invention comprises, in the vicinity of the bypass diode, means for preventing the temperature rise of a bypass diode resulting from a cause other than a current.
By employing this module and measuring the temperature rise in the bypass diode with the above-described method, it is possible to avoid the difficulty in the failure detection resulting from a fact that the bypass diode becomes excessively heated, for example, by the direct sunshine and the specific heat generated by the bypass diode itself can not be discriminated.